1. Field of the Invention
The present invention relates generally to an apparatus for inspecting semiconductor devices, and more particularly to an improved optical inspection system for the efficient and highly accurate determination of various measurement characteristics of semiconductor devices, such as lead spacing, coplanarity, and tweeze.
2. General Description of Prior Art
The inception and advancement of semiconductor devices has induced a plethora of related technologies and associated product demands. Many of these technologies concern themselves with the inspection of packaged integrated circuits and similar devices in order to determine product compliance within specific ranges of manufacturing and operational parameters. An important segment of these parameters involves ensuring that these devices will make proper electrical contact with a circuit board to which they are normally mounted. Typical examples of such devices are PLCCs, SOJs, SOICs, etc.
Manual inspection of the devices can be achieved by placing the device on a smooth surface and noting any separation between the contact points of any lead from the device and the surface. Visual inspection can also be made by sighting down the row of contact pins and noting excessive variation. However, these types of manual inspections are time consuming and not highly reliable, because of the tolerances and limits involved in the necessary measurements.
A typical example of an apparatus designed to accurately inspect one parameter of packaged semiconductor devices, coplanarity, is disclosed in U.S Pat. No. 4,754,555. The apparatus described therein employs a socket assembly for receiving the packaged semiconductor device and is operative to contact each lead of the package that is within acceptable coplanarity limits and indicate those leads that are not within acceptable limits. Although the device provides accurate coplanarity measurements within certain limits, it has inherent limitations in its use with fine pitch devices. Additionally, this apparatus is effective solely in the measurement of coplanarity, only one of a number of measurements that need to be made in order to ensure proper device function.
Significant improvements in measurement capabilities were achieved by the introduction of an optical scanning apparatus called the SMD 9000 Inspection System, manufactured by Trigon/Adcotech of Milpitas, California. This system uses a multi-camera optical array to determine specific precision measurements for the semiconductor device characteristics of tweeze, lead spacing, and coplanarity. In this apparatus, a semiconductor device is placed on platform that has a mirrored finish. Four cameras are placed facing the sides of the device, and one is placed overhead. The overhead camera is used to measure tweeze, and the four cameras facing the sides of the device are used to measure lead spacing and coplanarity of each of the respective sides. The device is capable of providing highly accurate measurements of tweeze, coplanarity, and lead spacing, even when used with fine pitch devices. However, the optical system components are expensive and highly sensitive. Each camera must be preserved in perfect alignment in order to ensure accurate device measurements. Additionally, the same power must be maintained for each of the respective cameras, and there are unavoidable problems with light intensity and distribution.